This invention relates to an image forming apparatus, such as a copying apparatus, for forming an image on an image bearing member in accordance with an image of a document placed on a platen.
As is well known, in recent years automatic document feeders have been developed in which a document is automatically placed in the exposure position on a platen. After exposure has been completed, the document is automatically discharged from the platen. High speed feeding operation in such an apparatus is a desirable feature.
One automatic document feeder which fulfills this high speed requirement is disclosed in Japanese Patent Disclosure (Kokai) No. 2-10384, Watanabe et al. In an automatic document feeder of this type, placement of the document at the exposure position and extraction of the document after completion of exposure are performed using a feed roll provided at the end of the platen.
However, in such a device, the mechanism for positioning the document in the exposure position is arranged at the feed roll end of the document placement platen. Moreover, the starting position for scanning by a scanning device that exposes and scans the document is set at an end on the opposite side of a reference position of the platen. As a result, when the size of the document is smaller than A4-crosswise, the scanning device scans over the half length of the document placement platen from the starting position of scanning at the end of the platen, irrespective of the document size. This means, for example, when the size of the document is B5-crosswise and the size of the copying paper is also B5-crosswise, the image of the document formed on the copying paper is shifted. Another problem is that the exposure and scanning time, which is longer than necessary, reduces the ability to increase the speed of image formation. In addition, the excess scanning wastes electric power.
Thus, conventional scanning devices scan the half length of the document placement platen, irrespective of the size of the document placed on the platen. As a result, when the size of the document and the size of the copying paper are smaller than the scanning length of the scanning device, the image of the document that is formed on the copying paper is shifted and part of the image of the document fails to be formed on the copying paper. Furthermore, in the case of small size documents, excess exposure and scanning time is required. This presents an obstacle to increasing the speed of copying.